A gas generator for quickly expanding and inflating an airbag is installed in an airbag module which is installed in a steering wheel or an instrument panel for protecting a vehicle occupant from the impact resulting from an automobile collision. Then, the gas generator actuates an igniter (squib) in response to an electrical signal from a control unit (actuator) to burn a gas generant by a flame of this igniter for quickly generating a large quantity of gas.
In the case of conventional designs, the gas generator always makes an airbag quickly be expanded and inflated irrespective of the sitting position of the vehicle occupant (a normal sitting posture or an irregular sitting posture such as a stoop) and irrespective of the speed (acceleration) of the vehicle at the time of collision. Accordingly, it is difficult to control the inflation of an airbag in accordance with the sitting position of the vehicle occupant and the speed (acceleration) of the vehicle at the time of collision so that the airbag may not fulfill its primary function of protecting the vehicle occupant.
Hence, in recent years, situation-adaptive gas generators for airbag have been proposed and developed to control the inflation of an airbag in accordance with the sitting position of the vehicle occupant and the speed (acceleration) of the vehicle at the time of collision, for example, to provide a moderate inflation of the airbag at the first stage.
For example, U.S. Pat. No. 6,032,979 discloses a situation-adaptive gas generator in which are formed two combustion chambers, each having its own igniter, so that the gas generants loaded into the respective combustion chambers are burned one after the other. This is provided with a filter element as an annular member located along the inner peripheral portion of a housing, a first combustion chamber located inside the filter element and, in addition to this, a second combustion chamber located in the first combustion chamber and partitioned by a cylindrical tube with base in order that the first combustion chamber and the second combustion chamber can communicate through a plurality of orifices formed in the cylindrical tube with base. By this configuration, the situation-adaptive gas generator can operate suitably for the sitting position of the vehicle occupant by burning the respective gas generants loaded into the first combustion chamber and the second combustion chamber with a time difference.
However, because the first combustion chamber and the second combustion chamber can communicate with each other, the gas generated in the first combustion chamber can make the gas generant loaded into the second combustion chamber begin to burn with the fear of not generating gas with the predetermined time difference. In addition, the filter element is provided as an annular member located along the inner peripheral surface of the housing so that the gas and residue generated by the combustion in the respective combustion chambers are cooled and filtered by the single filter. Because of this, when the ignition pattern between the two combustion chambers is a simultaneous ignition, a staggered ignition or a single sided ignition, the gas as output may become thermally and/or compositionally inappropriate due to over- or under-cooling through the filter. Furthermore, the ignition order of the combustion chambers is fixed to the order from the outer peripheral side to the inner peripheral side, and therefore it may not be applied to a variety of collision patterns.
On the other hand, Japanese Patent Published Application No. Hei 11-59318 discloses a situation-adaptive gas generator with a housing whose inside space are partitioned to form two combustion chambers in the upper and lower sides in the x-axis direction with an igniter provided for each combustion chamber so that the gas generants loaded into the respective combustion chambers are burned one after the other to output gas. This is costly because of the annular filter elements provided in the respective combustion chambers.
It is an object of the present invention to provide, at a low cost, a situation-adaptive gas generator having a combustion chamber which is partitioned into two or more chambers, and filter elements which can be independently used.